Facing a barrage of questions he could not answer from his Senate Education Committee colleagues — particularly about the right of California children to attend public schools even if they are unvaccinated — Sen. Richard Pan on Wednesday agreed to delay by one week the committee’s vote on his controversial vaccine legislation.

The unexpected retreat seemed a promising turn of events for hundreds of opponents who again showed up in the Capitol to challenge lawmakers and insist the bill would deprive them of their right to choose not to vaccinate their children. And some believe the delay could imperil the chances of the legislation’s passage.

Pan, a pediatrician, co-authored Senate Bill 277, which would repeal the state’s personal belief exemption and require that only children who have been immunized for diseases such as measles and whooping cough be admitted to a school in California. It would also require schools to notify parents of immunization rates at their children’s schools.